Closure On Skepticism

Journal of Philosophy 107 (5):243-256 (2010)
It is received wisdom that the skeptic has a devastating line of argument in the following. You probably think, he says, that you know that you have hands. But if you knew that you had hands, then you would also know that you were not a brain in a vat, a brain suspended in fluid with electrodes feeding you perfectly coordinated impressions that are generated by a supercomputer, of a world that looks and moves just like this one. You would know you weren’t in this state if you knew you had hands, since having hands implies you are no brain in a vat. You obviously don’t know you’re not a brain in a vat, though—you have no evidence that would distinguish that state from the normal one you think you’re in. Therefore, by modus tollens, you don’t know you have hands. At least, the skeptic has a devastating argument, it is thought, if we grant him closure of knowledge under known implication, which many of us are inclined to do: roughly, if you know p, and you know that p implies q, then you know q.i To say that this is an intuitively compelling argument is an understatement; the project of finding a reply that isn’t table-thumping, or obfuscating, or special-pleading has exercised philosophers for a very long time. The steps of the argument have been scoured in detail to try to find cracks that will yield under pressure. Some of these efforts have been intriguing, and illuminating, and some even appear to provide dialectical victories that shift the burden of proof back to the skeptic. However, as refutations they all come up short. I will argue that we have missed a very simple point: though the skeptical argument above is valid, it has a false premise, namely, the claim that the thing we obviously know implies the thing we seem obviously not to know. This premise, I will argue, cannot be repaired, so we have a refutation; if the skeptic wants to convince us to worry about our ordinary knowledge, he will have to come up with a completely different argument. Closure of knowledge under known implication (hereafter “closure”), is obviously necessary for the skeptical argument presented above..
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